A conventionally known technique involves changing optical frequency of output laser light by changing the drive current of a semiconductor laser. When the drive current of the semiconductor laser is changed in this way, the intensity of the output light also changes. Therefore, in order to maintain constant output light, an optical amplifier is provided with a feedback control function to monitor the intensity of the output light and control the amplification factor according to a change in the output light, as shown in Patent Document 1, for example.    Patent Document 1: Japanese Patent Publication Application No. H6-196791